(navigation image)
Home American Libraries | Canadian Libraries | Universal Library | Community Texts | Project Gutenberg | Children's Library | Biodiversity Heritage Library | Additional Collections
Search: Advanced Search
Anonymous User (login or join us)
Upload
See other formats

Full text of "The Cambridge Natural History"

60                            MULTITUBERCULAR TEETtt                        CHAP.
variety of Orders. The same condition, as has been noted, char-
acterises that ancient Ungulate form JSuprotogonia. Even where
the teeth seem at first sight to be tritubercular a detailed study
shows traces of otherwise vanished cusps.
It must be remembered in basing arguments upon the early
Jurassic and Cretaceous mammals, that our knowledge of them
mainly depends upon lower jaws, the teeth of which are usually
simpler in pattern than those of the upper jaws. Moreover,
another fact, not always insisted upon, must not be lost sight of,
In many of those creatures the jaws were of small size, and yet
accommodated a large series of molar teeth. ^mpJiitherium, for
example, had six molar teeth, and five is a number frequently
met with. As the teeth are so numerous and the jaws so small
it seems reasonable to connect the simplicity of the structure of
the teeth with the need for crowding a number together. The
same argument may partly account for the superabundant teeth
of many Toothed Whales, It is true that the Manatee has very
numerous grinders which are yet complex; but then in this
animal there is a succession, and the jaw does not hold at a
given time the entire series, with which it is provided in relays.
On the other hand, where there are few molars they are often
of the multitxibercular type, or at least approach it; of this
the Multitubereulate Poly"mastodon is a good example ; so, too, the
molars of Hydrochoerus, and of many other Rodents.
It is well known that the fourth deciduous molar of the
upper jaw, which is replaced by a permanent premolar in the
fully adult animal, is of a more complex structure than its
successor. This may indeed be extended to premolars earlier in
the series. In the Dog " the second and first milk molars closely
resemble the third and second premolars " ; now the milk premolars
belong evidently to the same dentition as the permanent molars,
and they are earlier teeth than the later-developed replacing
teeth. It is therefore significant that these earlier teeth should
be more cuspidate than the later teeth. It tolls distinctly in
favour of thİ simplification as opposed to the complication of
teeth in time, in the groups concerned.
These facts may possibly be applied in explanation of the
simple teeth of some of the Jurassic and Cretocoous mammals.
It has been, mentioned that absolute trituberculy is exceedingly
rare among those ancient creatures; more generally there are to